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Normally, the molecules in the waterfall move under the influence of both the flow and thermal agitation. But if it gets cold enough, the heat effect becomes so low that not even the waterfall's motion can stop ice forming, gradually at first but then ever more rapidly.

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Pretty much, it'll freeze from the bottom up as ice forms and piles on and forms a sort of column until it reaches to the top of the waterfall where it meets with the frozen top layer.

This video shows it in progress, you can see the columns forming here: https://youtu.be/nWWWHQ7_MQQ

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Just as a side note: Why was this not your initial answer?

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I've not really answered posts like these before, so not sure how much detail people would have liked/disliked. :)

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The closest big frozen falls to me seem to slowly form in the first few cold snaps, a bit at a time, and by January this year, look like stalactites.

I did notice there’s a sheen of not yet frozen on it, so it’s making layers.

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